1. Field of Invention
This invention pertains to a gas-fired heating mantle for heating a retort furnace, and more particularly to a heating mantle with a porous wall disposed in the path for the combustion gases for raising the efficiency of heat transfer to the furnace.
2. Description of the Prior Art
Gas-fired heating mantles are used extensively in the metal processing industry for treating and processing metals and alloys, as well as in the inorganic chemical industry in reactors. However present mantles are severely deficient in a number of areas which limits their use in commercial applications. The primary deficiency of present heating mantles is limited heat transfer rate from the mantle to the retort.
Typically, a gas-fired heat mantle surrounds a furnace retort vessel, and is constructed to provide a high rate of heating in a small space.
Typically, the mantle is made of a steel shell with an inside lining of insulating refractory and must be shaped to direct combustion flames away from the retort vessel to avoid damaging it. In this configuration, heat is transferred to the retort primarily through two mechanisms: one, by convective heat transfer from the combustion gases to the interior mantle wall and the retort vessel wall, and two, by radiation from the interior mantle wall to the retort vessel wall. In a gas-fired heating mantle, at temperatures below 1200.degree. F., the radiation heat transfer rates are low due to lower temperatures, and the convective heat transfer rates are generally low due to low gas velocities. This combination results in low overall heat transfer rates.
At temperatures above 1400.degree. F., heat transfer by radiation from the mantle wall occurs at high rates, however, the convective rates to the heating mantle wall remain low and becomes the rate limiting step in the overall heat transfer process. This keeps the overall heat transfer rates low.
Typically, present heating mantles have a heat transfer rate in the range of 5-15 BTU/sq. ft.-hr.-degree F. depending upon temperature level and gas flow rates.